Zero insertion force sockets using negative thermal expansion materials

ABSTRACT

A socket device for receiving a connection pin is disclosed, the socket device including a substrate having an upper surface. The socket device includes a connection pad disposed on the upper surface and a first layer disposed on the upper surface and on the connection pad. The first layer includes material having an overall positive coefficient of thermal expansion. The socket device includes a second layer disposed on the first layer. The second layer includes material having an overall negative coefficient of thermal expansion. The socket device also includes a contact hole formed in the first and second layers exposing a portion of the connection pad.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of integrated circuits,and, more particularly, to integrated circuit devices having offsetjunctions to protect circuits from an electrostatic discharge (ESD) andmethods for their manufacture.

2. Description of the Related Art

Electrical components are often mounted in sockets comprising springcontacts. These sockets allow easy installation and replacement ofelectrical elements. With conventional sockets, the terminal pins of anintegrated circuit (IC) are pressed into a socket and deflect the springcontacts.

Modern integrated circuits are often complex and may have a large numberof pins. While the force required to insert an individual pin into asocket receptacle is modest, simultaneously inserting a large number ofpins into their respective sockets can require a significant insertionforce. The insertion force can damage the integrated circuit or bend thepins when the force is significant or when the sockets and pins are notproperly aligned.

To avoid the problem of damaging the IC or the pins, various kinds ofzero insertion force (ZIF) sockets have been developed, whereby theterminal pins of an IC can be inserted into and withdrawn from suchsockets with minimal or no insertion and withdrawal force. Thisfacilitates easy change or replacement of the IC, which is oftennecessary.

A widely used conventional zero insertion force socket substantiallyincludes a socket housing having a plurality of metallic conductivecontacts disposed in the insertion holes. The insertion holes of thesocket are adapted to receive multiple terminal pins of an IC. Accordingto the zero insertion force arrangement, during the insertion andwithdrawal of the IC from the socket, the IC will suffer little or noresistant force from the contacts of the socket. After the pins areinserted into the insertion holes, an operation lever is moved to push amovable plate disposed over the socket housing. The plate then slides asmall distance (usually 1 mm) relative to the socket housing, wherebythe terminal pins are urged to move toward the contacts and squeeze intoa space between two opposite conductive elastic plates comprising thecontacts.

Several shortcomings may be perceived in the conventional structure.Typical ZIF sockets are advantageously used to receive large dies orpackages, for example a Pentium™ chip. For smaller-scale socket devices,for example a "flip chip" assembly, the dies or packages are too smallto be mechanically processed practically with a through-hole type ZIFsocket employing the offset method discussed above. In addition, as thepackage size becomes smaller, the proximity of the pins to one anotherbecomes so close that the pins sometimes come into contact with oneanother and short the circuit.

The present invention is directed to overcoming, or at least reducingthe effects of, one or more of the issues set forth above.

SUMMARY OF INVENTION

In accordance with one aspect of the present invention, a socket devicefor receiving an IC terminal pin or solder ball is provided. The socketdevice includes a substrate having an upper surface, a connection paddisposed on the upper surface of the substrate and a first layerdisposed on the upper surface and on the connection pad. The first layerincludes material having an overall positive coefficient of thermalexpansion. The socket device includes a second layer disposed on thefirst layer. The second layer includes material having an overallnegative coefficient of thermal expansion. The socket device alsoincludes a contact hole formed in the first and second layers exposing aportion of the connection pad which is receptive of a pin. In analternative embodiment the socket device only includes the negativecoefficient of thermal expansion layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and aspects of the invention willbecome further apparent upon reading the following detailed descriptionand upon reference to the drawings in which:

FIG. 1 is a cross-sectional view of the socket device at roomtemperature;

FIG. 2a is a cross-sectional view of the socket device at an elevatedtemperature with a pin inserted into the socket;

FIG. 2b is a cross-sectional view of the socket device at normaloperating temperature with the pin inserted into the socket;

FIG. 3 is a cross-sectional view of the socket device at normaloperating temperature with a solder ball/die inserted into the socket.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, that will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Turning now to the drawings, and in particular to FIG. 1, one embodimentof the socket device in accordance with the present invention isdisclosed. The zero insertion force socket 10 comprises a substrate 12with an upper surface 14, a connection pad 16, a first layer 18 disposedon the substrate 12 and connection pad 16, and a second layer 20disposed on the first layer 18.

Substrate 12, for example a ceramic or organic substrate, comprises abase of the socket device 10 upon which the socket is built. Substrate12 may also comprise a die/package of another socket assembly upon whichthe next socket is built. Connection pad 16 is disposed on the uppersurface of substrate 12 and attached thereto by a photo define/etchmethod. Also disposed on the upper surface of substrate 12 and onconnection pad 16 is first layer 18. First layer 18 is bonded tosubstrate 12 in a way similar to the connection pad by a spin-on andphoto define/etch method or drilling. In accordance with one aspect ofthe invention first layer 18 comprises material with an overall positivecoefficient of thermal expansion, indicating that the material willexpand as temperature is elevated and contract as temperature decreases.In one embodiment, first layer 18 comprises silicon oxide SiO₂, siliconnitride Si₃ N₄, a polyimide, or similar materials. When substrate 12comprises ceramic or organic material, first layer 18 preferablycomprises polyimide or other organic material. However, when substrate12 comprises a die or package, first layer 18 preferably comprisessilicon oxide or silicon nitride. In an alternative embodiment, firstlayer 18 may be absent from the socket device.

Disposed on the upper surface of first layer 18 is second layer 20. Inaccordance with another aspect of the invention, second layer 20comprises materials with an overall negative coefficient of thermalexpansion, indicating that the second layer contracts with a temperatureelevation and expands with a temperature decrease. The second layermaterial preferably has a wide temperature range for which the materialexhibits the negative expansion behavior, in order to accommodate use ofthe socket device in apparatuses with various operation temperatures.One material exhibiting the behavior desirable for the purposes ofpracticing the present invention is zirconium tungstate ZrW₂ O₈. Thezirconium tungstate may be a single crystal ZrW₂ O₈, an amorphous ZrW₂O₈, or a polymer bound ZrW₂ O₈. Those of ordinary skill in the art willappreciate that zirconium tungstate exhibits a negative coefficient ofthermal expansion over the desired wide range of temperatures, from nearabsolute zero to 1050° K., its decomposition temperature. Second layer20 is bonded to first layer 18 with a compliant interfacial layer 22that allows first layer 18 and second layer 20 to remain bonded to oneanother despite the opposite tendencies of the layers to contract andexpand with temperature changes. The interfacial layer 22 material, forexample Hitachi's HF-X20, is such that it absorbs the stress between thefirst and second layers as the opposing movement between layers occurs.

Both first and second layers 18 and 20, respectively, exhibit aconnection hole 24 therethrough to expose a portion of the surface ofconnection pad 16. Connection hole 24 is etched using currenttechnologies, such as photo lithography or laser drilling. Connectionhole 24 is receptive of an integrated circuit pin 26 with zero insertionforce at elevated temperatures. FIG. 2a shows the socket in an elevatedtemperature condition with pin 26 inserted into connection hole 24. Theportion of connection hole 24 formed in first layer 18 has a lineardimension that is larger than the portion of connection hole 24 formedin second layer 20, the advantage of which being discussed in furtherdetail below. Connection hole 24 through both layers 18 and 20 issubstantially larger in dimension than pin 26 when the socket is atelevated temperatures. Connection hole 24 being larger than pin 26ensures that as pin 26 is inserted or retracted there are no forcesrequired that could potentially damage the pin or the associatedintegrated circuit. Connection hole 24 is chamfered at the pointdesignated with reference numeral 28, around the upper perimeter ofsecond layer 20 to act as self-guidance during pin insertion. Chamfer 28may be accomplished by facet etching or other methods, as would beappreciated by those of ordinary skill in the art.

Operation of socket device 10 is as follows: socket device 10 is raisedto an elevated temperature in anticipation of receiving pin 26. Thetemperature to which the socket device is elevated is such that it issubstantially higher than the temperature at which the device willultimately be operated. For example, if the socket device is intended tobe used in a CPU that has a normal operating temperature of 100° C., thetemperature to which the socket might be brought prior to insertion ofthe pin might be 200-250° C. When socket 10 is raised to an elevatedtemperature, second layer 20 contracts, enlarging connection hole 24through second layer 20. The elevated temperature has the oppositeeffect for connection hole 24 through first layer 18 of the device. Thehigher temperature causes first layer 18 to expand and thereforedecreases the size of connection hole 24 through the first layer.Because connection hole 24 through first layer 18 becomes smaller withelevated temperatures, connection hole 24 through first layer 18 has alarger dimension at operating temperatures than connection hole 24through second layer 20, to ensure a hole through both layers 18 and 20large enough to accommodate receipt of pin 26 without any insertionforce at elevated temperatures. This larger dimension of connection hole24 through first layer 18 may continue to be exhibited at elevatedtemperatures if the mounting temperature is greater than the operatingtemperature.

When an elevated temperature for the socket device 10 has been achieved,the socket can receive pin 26 with zero insertion force. FIG. 2aexhibits socket device 10 with pin 26 inserted while the device is in anelevated temperature condition. The insertion of pin 26 in this elevatedtemperature environment leaves pin 26 in substantially close proximityto connection pad 16, but short of full contact, as shown in FIG. 2a.

Following the insertion of pin 26, the socket and pin combination areremoved from the elevated temperature condition. As the environmenttemperature decreases, second layer 20, which has a surface in directcontact with the lower temperature medium, begins to cool. Second layer20 is subject to cooling by two modes of heat transfer: Conduction viadirect contact with the cooler environment, and convection, as naturalor forced convection currents are established. First layer 18 will alsobegin to cool by the conduction mode of heat transfer as second layer20, with which first layer 18 is bonded, continues to dissipate heat.However, first layer 18 will not be cooled by convection since it isinsulated from the lower temperature environment by a boundary layercomprising second layer 20. Thus second layer 20 will cool more quicklythan first layer 18 because first layer 18 only cools as it transfersheat conductively to relatively cooler second layer 20. As second layer20 cools, connection hole 24 through second layer 20 becomes smaller,due to the negative coefficient of thermal expansion behavior exhibitedby the second layer material. When second layer 20 cools sufficiently,the inner diameter of connection hole 24 through second layer 20 willmake contact with the circumferential surface of pin 26. The shrinkingof connection hole 24 through second layer 20 continues and pin 26 issecured within socket 10. Connection hole 24 is sized so as to securepin 26 in place at temperatures substantially near or below the normaloperating temperature of the equipment into which the IC and socket areplaced.

After pin 26 has been secured in place, as shown in FIG. 2b, the coolingof socket device 10 continues. First layer 18, which has been slower tocool than second layer 20, begins to contract in accordance with amaterial having a positive coefficient of thermal expansion in a coolerenvironment. The results of first layer 18 contraction are a largerconnection hole 24 through first layer 18, an increase in the annularspace between the circumferential surface of pin 26 and the innersurface of connection hole 24, and a reduction in the thickness of firstlayer 18. When the dimensions of first layer 18 reduce, second layer 20moves into closer proximity with substrate 12. This occurs becausesecond layer 20 is bonded to first layer 18. The movement of secondlayer 20 toward the upper surface of substrate 14 concurrently urges pin26, which is secured within connection hole 24, into contact withconnection pad 16. Socket insertion is complete upon contact between pin26 and connection pad 16.

If it becomes desirable to remove pin 26 from the socket, the socket/pinassembly need only be re-elevated to a temperature such that connectionhole 24 in second layer 20 enlarges and releases pin 26. Pin 26 can thenbe retracted from the socket with zero retraction force. The cycle ofheating and cooling to allow for insertion and retraction pins isindefinitely repeatable.

In an alternative embodiment depicted in FIG. 3, a socket device 10' isreceptive of a solder ball 30 connected to a die 32 instead of a pin.Those of ordinary skill in the art having the benefit of the presentdisclosure will appreciate that the insertion and retraction of solderball 30 comprises the same operation as for pin 26 described above.

While the present invention has been particularly shown and describedwith reference to various illustrative embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention. The above-described embodiments are intended to be merelyillustrative, and should not be considered as limiting the scope of thepresent invention.

What is claimed is:
 1. A socket device for receiving a connection pin orsolder ball, said socket device comprising:a substrate having an uppersurface; a connection pad disposed on said upper surface; a first layerdisposed above said upper surface, said first layer includingmaterialhaving an overall negative coefficient of thermal expansion; and acontact hole formed in said first layer exposing a portion of saidconnection pad.
 2. A socket device for receiving a connection pin orsolder ball, said socket device comprising:a substrate having an uppersurface; a connection pad disposed on said upper surface; a first layerdisposed on said upper surface and on said connection pad, said firstlayer including material having an overall positive coefficient ofthermal expansion; a second layer disposed on said first layer, saidsecond layer including material having an overall negative coefficientof thermal expansion; and a contact hole formed in said first and secondlayers exposing a portion of said connection pad.
 3. The socket deviceof claim 2, including a bonding layer disposed between said first andsecond layers, said bonding layer bonding said first and second layerstogether.
 4. The socket device of claim 2, wherein said contact holeincludes a first portion formed in said first layer having a firstlinear dimension and a second portion formed in said second layer havinga second linear dimension smaller than said first linear dimension. 5.The socket device of claim 1, wherein said layer includes zirconiumtungstate.
 6. The socket device of claim 2, wherein said second layerincludes zirconium tungstate.
 7. The socket device of claim 5, whereinsaid layer including zirconium tungstate has a substantially isotropicnegative thermal expansion behavior.
 8. The socket device of claim 6,wherein said second layer including zirconium tungstate has asubstantially isotropic negative thermal expansion behavior.
 9. Thesocket device of claim 7 or 8, wherein said substantially isotropicnegative thermal expansion behavior is exhibited at least in atemperature range of from about 100° C. to about 200° C.
 10. The socketdevice of claim 9, wherein said substantially isotropic negative thermalexpansion behavior is exhibited at least at a temperature of about 150°C.
 11. The socket device of claim 1, wherein said contact hole includesa chamfer formed in said layer.
 12. The socket device of claim 2,wherein said contact hole includes a chamfer formed in said secondlayer.
 13. The socket device of claim 3, wherein said contact holeincludes a first portion formed in said first layer having a firstlinear dimension and a second portion formed in said second layer havinga second linear dimension smaller than said first linear dimension. 14.The socket device of claim 3, wherein said second layer includeszirconium tungstate.
 15. The socket device of claim 14, wherein saidsecond layer including zirconium tungstate has a substantially isotropicnegative thermal expansion behavior.
 16. The socket device of claim 15,wherein said substantially isotropic negative thermal expansion behavioris exhibited at least in a temperature range of from about 100° C. toabout 200° C.
 17. The socket device of claim 16, wherein saidsubstantially isotropic negative thermal expansion behavior is exhibitedat least at a temperature of about 150° C.
 18. The socket device ofclaim 1, further comprising a second layer disposed between said firstlayer and said upper surface.